Disease and trauma affecting the articular surfaces of a joint, such as the knee joint, often are treated by surgically replacing the articular surfaces of the bones that meet at the joint, such as the femur and tibia of the knee joint, with prosthetic femoral and tibial implants, referred to as total knee replacements (“TKR”).
In total knee replacement (TKR) surgery, a surgeon typically will surgically resect the distal end of the patient's femur (typically at a base angle orthogonal to the mechanical axis of the leg as measured in an A-P X-ray) and replace it with a femoral articular component.
The femoral articular component may be mounted directly to the resected end of the femur or, for added strength and bonding, may be mounted on a femoral stem component that is inserted into an intramedullary canal drilled into the femur.
The femoral articular component will commonly comprise a medial condylar articular surface and a lateral condylar articular surface separated by at least a partially open space, thereby simulating the distal end of a natural femur. The medial and lateral condylar articular surfaces of the femoral articular component replace and serve the function of the medial and lateral condyles of the natural knee.
The surgeon also will surgically resect the proximal end of the tibia (also, typically perpendicular to the mechanical axis of the leg as measured in an A-P X-ray, even though the plateau at the proximal end of a natural tibia typically has is slightly varus angulated) and replace it with a tibial articular component comprising a tibial articular surface. Usually, the tibial implant includes a tibial stem component that extends at an angle to a tibial plateau. The stem will extend into a surgically formed opening in the patient's intramedullary canal of the tibia. The stem component may be formed of titanium or cobalt chromium alloys compatible with the tibial tray implant or another biocompatible metal. A plastic or polymeric (often ultra high molecular weight polyethylene) insert (or bearing) articular component is placed atop the plateau at the proximal end of the stem component and comprises the tibial articular surfaces upon which the condylar articular surfaces of the femoral articular component will ride.
The tibial insert commonly will comprise medial and lateral compartments comprising medial and lateral articular surfaces that mate with the medial and lateral condyles on the femoral articular component, respectively. The medial and lateral articular surfaces on the tibial articular component replace and serve the function of the medial and lateral menisci of the natural knee.
One or both of the femoral and tibial articular surfaces typically are made of biocompatible, low friction, hard material, such as cobalt chromuim or a polymer, such as a high molecular weight polyethylene.
In a natural knee, the relative movement of the femur, tibia, and patella (the knee cap) is a complex combination of flexion-extension, translation, and rotational movements in all six degrees of freedom and is difficult to emulate with a prosthesis. For instance, in a natural knee, the condyles of the femur translate relative to the tibial bone as well as rotate. The overall translation is much greater in the lateral compartment than in the medial compartment (e.g., from 0 to 120 degrees of flexion, the tibio-femoral contact point moves on average about 20 mm on the lateral side and about 12 mm on the medial side. (See Walker P S, Heller Y, Yildirim G, Immerman I.: Reference axes for comparing the motion of knee replacements with the anatomic knee. Knee. 2011 October; 18(5):312-6. Epub 2010 Aug. 17.)) Intracapsular anatomical structures such as the ACL (Anterior Cruciate Ligament), PCL (Posterior Cruciate Ligament), menisci, the bone anatomy, and the muscles acting on the knee joint influence a phenomenon called the screw home mechanism. Particularly, the two rounded ends (condyles) of the femur have different radii. During knee extension, the tibia glides anteriorly on the femur. Through the last 20 degrees of knee extension, anterior tibial glide persists on the tibia's medial condyle in part because the medial tibial articular surface is longer in the anterior-posterior dimension than the lateral tibial articular surface and in part due to the tension forces applied by the muscles and ligamentous stabilizers. This produces external tibial rotation, i.e., the “screw-home” mechanism.
In the final phase of extension, as the knee enters its final few habitual degrees of extension or hyperextension, the anterior cruciate ligament as well as both collateral ligaments are taut, and the knee is in its maximally stable position, with the leg able to support body weight despite the quad muscles being completely relaxed.
The full range of motion of a natural knee commonly is in the range of about 10°-15° of hyperextension up to about 140° to 155° of flexion and maximally up to about 165° in certain individuals. Most of the tibial rotation in lower flexion activities such as walking or stair climb occurs in the first 0-30° of flexion (70%). The knee has little rotational freedom in extension, thereby providing stability with low energy expenditure while standing. However, in natural deep flexion knee motion, there typically is an internal rotation of the tibia relative to the femur at angles of significant flexion of the knee joint, about 90° of flexion and greater. See Hollister, A. M., Jatana, S., Singh, A. K., Sullivan, W. W., and Lupichuk, A. G.: The Axes of Rotation of the Knee, Clin. Orthop. Relat. Res., 290, pp. 259-268; Roland M, Hull M L, Howell S M.: Virtual axis finder: a new method to determine the two kinematic axes of rotation for the tibio-femoral joint. J Biomech Eng. 2010 January; 132(1):011009; Hefzy M S, Kelly B P, Cooke T D, al-Baddah A M, Harrison L.: Knee kinematics in-vivo of kneeling in deep flexion examined by bi-planar radiographs. Biomed Sci. Instrum. 1997; 33:453-8; and Spanu C E, Hefzy M S.: Biomechanics of the knee joint in deep flexion: a prelude to a total knee replacement that allows for maximum flexion. Technol. Health Care. 2003; 11(3):161-81
By way of definition, internal rotation of the tibia refers to the tibia rotating about its longitudinal axis medially from an anterior perspective. For instance, looking down on one's own right tibia, in deeper flexion, the tibia rotates counterclockwise relative to the femur as one flexes the knee in the direction of flexion. This internal rotation facilitates deep knee flexion by providing clearance for the soft tissue, e.g., ligaments, tendons, and muscles, surrounding the knee joint.
Additionally, the natural menisci of the knee are formed substantially of soft tissue such as cartilage and, therefore, are compressible, i.e., they can change shape. The menisci in a natural knee also move relative to the tibia during movement of the knee. However, current technology and materials for prosthetic knee articular components are not sufficiently advanced to provide shape change functionality. Rather, in prosthetic knees, the tibial articular surfaces (the replacement of the natural menisci) generally are made of a hard polymer such as polyethylene and are relatively stiff under physiologic loads. Also, most are fixedly attached to the tibia. The shape of the articulating surfaces, including the femoral condyles and the tibial plateau and the locations where the femur contacts the tibia versus the horns of the meniscii, have been nicely summarized by Freeman M A, Pinskerova V.: The movement of the normal tibio-femoral joint. J. Biomech. 2005 February; 38(2):197-208, pp 201-203.
Freeman et al also summarized the tibio-femoral rotational characteristics from full extension to deep flexion (pp 203-206).
Also, most TKR implants do not provide an extension-flexion range anywhere near the roughly 165° range of motion of a natural knee. Most TKR implants at this time provide a range of motion of about 10° of hyperextension to about 115° to 130° degrees of flexion, which is less than what is necessary or desirable for many common daily activities, such as kneeling, squatting, and certain sports activities.